In one form the present invention provides an apparatus in an airflow path before a particle detector, wherein the apparatus removes a substantially constant proportion of all sizes of airborne particles from the airflow over time. In an example the apparatus includes a flow splitting arrangement configured to divide a fluid flow into a plurality of sub-flows, the splitting arrangement 10 including means for defining a plurality of substantially identically dimensioned flow apertures configured to direct a portion of the fluid into a respective sub-flow.